1. Field of the Invention
The present invention relates to an improvement in a process for beam-welding two members different in hardness from each other, that is, high and low in hardness, wherein joint surfaces of the high-hardness and low-hardness members are welded to each other by use of a laser beam or an electron beam.
2. Description of the Related Art
In the prior art, even when joint surfaces of high-hardness and low-hardness members are welded to each other by use of a laser beam or an electron beam, a point of irradiation of the laser beam or the electron beam is set on the joint surfaces of the two members, similarly as when two members having the same hardness are welded to each other.
However, in the welding of the joint surfaces of the high-hardness and low-hardness members to each other by use of the laser beam or electron beam, if the point of irradiation of the laser beam or electron beam is set on the joint surfaces of the two members, a poor weld often occurs for the following reasons:
(1) The heat input is too strong for the high-hardness member and for this reason, the high-hardness member is brought into a rehardened state, resulting in a cracking produced.
(2) Even if no cracking is produced immediately after the welding, a cracking may be produced in some cases due to a variation in temperature during service of the welded members.
(3) The melting of the high-hardness member during the welding is large and for this reason, a large amount of carbide is precipitated during solidification of the high-hardness member, which causes a cracking in the weld zone.
Accordingly, it is an object of the present invention to provide a process for beam-welding two members different in hardness from each other, wherein a poor weld as described above can be avoided to the utmost.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a process for joint surfaces of a high-hardness member and a low-hardness member are welded to each other by use of a laser beam or an electron beam, wherein a point of irradiation of the laser beam or the electron beam is set at a location offset from the joint surfaces of the high-hardness member and the low-hardness member toward the low-hardness member by a predetermined distance, so as to cause the melting provided by the beam to spread from the low-hardness member to the high-hardness member. The high-hardness member corresponds to a valve seat member 12 and a valve member 19 in an embodiment of the present invention, which will be described hereinafter, and the low-hardness member corresponds to a housing body 11 and a valve rod 18 in the embodiment.
With the first feature, the following merits can be achieved:
(1) when the welding using the laser beam or the electron beam is started, the melting first occurs in the low-hardness member and then spreads to a periphery of the low-hardness member to ultimately reach the high-hardness member. Therefore, the melting in the high-hardness member is advanced relatively slowly without reception of a direct heat input provided by the laser beam and hence, the high-hardness member is not brought into a rehardened state in which a cracking is liable to occur;
(2) the melting of the low-hardness member is larger than that of the high-hardness member and hence, the low-hardness member difficult to crack is molten into the high-hardness member, and a cracking-causing element in the high-hardness member is diluted with the low-hardness member; and
(3) the melting of the high-hardness member is smaller than that of the low-hardness member and hence, the variation in temperature up to the solidification of the high-hardness member is also relatively small and thus, the precipitation of a carbide from the high-hardness member can be inhibited.
Consequently, the joint zones of the high-hardness member and the low-hardness member can be welded, while avoiding a poor weld such as cracking of the high-hardness member to the utmost. Even during service of both the members, it is possible to prevent a cracking from occurring in the weld zones of the members.
According to a second aspect and feature of the present invention, in addition to the first feature, the high-hardness member is a spherical valve member of an electromagnetic fuel injection valve, and the low-hardness member is a valve rod welded to the valve member; and wherein the laser beam or the electron beam is emitted to the point of irradiation offset from the joint surfaces of the valve member and the valve rod toward the valve rod by the predetermined distance, so as to cause the melting provided by the beam to spread from the valve rod to the valve member.
With the second feature, the joint zones of the valve member and the valve rod of an electromagnetic fuel injection valve can be welded to each other, while avoiding a poor weld such as cracking to the utmost. In addition, even during service of the valve member and the valve rod, it is possible to prevent a cracking from occurring in the weld zones of the valve member and the valve rod.
According to a third aspect and feature of the present invention, in addition to the fist feature, the high-hardness member is a valve seat member of an electromagnetic fuel injection valve, and the low-hardness member is a valve housing body welded to a rear end of the valve seat member; and wherein the laser beam or the electron beam is emitted to the point of irradiation offset from the joint surfaces of the valve seat member and the valve housing body toward the valve housing body by the predetermined distance, so as to cause the melting provided by the beam to spread from the valve housing body to the valve seat member.
With the third feature, the joint zones of the valve seat member and the valve housing body of the electromagnetic fuel injection valve can be welded to each other, while avoiding a poor weld such as cracking to the utmost. In addition, even during service of the valve seat member and the valve housing body, it is possible to prevent a cracking from occurring in the weld zones of the valve seat member and the valve housing body.
According to a fourth aspect and feature of the present invention, in addition to any one of the first to third features, the distance of offsetting of the point of irradiation of the beam with respect to the joint surfaces is in a range of 0.5 to 1.5 mm.
With the fourth feature, the welding strengths of both the members can be ensured, while avoiding the cracking of the high-hardness member.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.